1. Field of the Invention
Embodiments of the present invention generally relate to energy storage devices and, more particularly, to flywheel energy storage devices.
2. Description of the Related Art
Society's ever increasing need for electricity requires an electrical grid that has a large electrical energy storage capacity. The larger capacity provides for improved grid efficiency and reliability, while providing high quality (sustained voltage) power. One type of energy storage device that provides promise for the electrical industry is flywheel-based energy storage devices.
Flywheel-based energy storage devices store energy in the form of the rotational inertia of one or more flywheels. Some of these devices temporarily store mechanical energy to remove fluctuations in the rotational velocity of an apparatus, such as an internal combustion engine. Other flywheel based energy storage devices have an electric motor to convert electrical energy into rotational mechanical energy, and an electric generator or alternator to convert the rotational mechanical energy into electrical energy. In some devices the electric motor and the electric generator or alternator are combined to form a motor/generator for both converting electrical energy into rotational mechanical energy, and converting the rotational mechanical energy into electrical energy.
One of the drawbacks in all flywheel based energy storage devices is that the capacity to store rotational mechanical energy is directly proportional to the mass of the flywheel. To provide systems capable of storing large amounts of energy, a flywheel with a proportionally large mass must be employed. This low energy to ratio weight (energy density) contributes to increased material cost, increased shipping costs and larger system footprint and weight.
Another concern common to flywheel based energy storage devices is stability, particularly at high rotational speeds. Instability can lead to vibrations resulting in energy loss through increased friction and material stress and fatigue that can ultimately result in structural failure.
As the demand for low-cost electrical energy increases, the efficiency of storing electrical energy in mechanical form takes on greater importance. Therefore, there is a need for an improved flywheel based energy storage apparatus with a higher energy density and increased stability.